Seismic evidence for subduction-transported water in the lower mantle

We use seismic attenuation tomography to identify a region at the top of the lower mantle that displays very high attenuation consistent with an elevated water content. Tomography inversions with >80,000 differential travel-time and attenuation measurements yield 3D whole-mantle models of shear velocity (V_S) and shear quality factor (Q_µ). The global attenuation pattern is dominated by the location of subducting lithosphere. The lowest Q_µ anomaly in the whole mantle is observed at the top of the lower mantle (660-1400 km depth) beneath eastern Asia. The anomaly occupies a large region overlying the high-Q_µ sheet-like features interpreted as subducted oceanic lithosphere. Seismic velocities decrease only slightly in this region, suggesting that water content best explains the anomaly. The subducting of Pacific oceanic lithosphere beneath eastern Asia likely remains cold enough to transport stable dense hydrous mineral phase D well into the lower mantle. We propose that the eventual decomposition of phase D due to increased temperature or pressure within the lower mantle floods the mantle with water, yielding a large low-Q_µ anomaly.